Many types of paper processing machines such as convenience copiers, printers, and the like, utilize paper feeding devices to move documents to a processing station. Machine operators may feed documents by inserting them one at a time into a feeding mechanism or may place a stack of documents upon a feed tray from which they are automatically fed. Blank sheets to be printed are generally fed to a print receiving station from a stack of sheets.
U.S. Pat. No. 4,052,054 describes a semiautomatic document feed device (SADF) for a convenience copier. In that device, the operator inserts a document to be copied into a pair of aligner rolls positioned near the front of the machine. Those rolls take the document from the operator's hand and move it forward to register the leading edge of the document against an entry gate. While moving the document forward, the aligner rolls simultaneously move the document sideways to a reference edge near the front of the machine so that when the document reaches the entry gate the side edge of the document is also in proper position. In the SADF shown in the above-mentioned patent, a second set of aligner rolls are provided near the rear of the machine such that the leading edge of a second document to be copied can be positioned at the entry gate in side-by-side relationship with a first document. As the second set of aligner rolls operate, the second document is side-edge referenced against a reference edge at the rear of the machine. When the entry gate drops, the aligner rolls are restarted to feed the two sheets to the processing station simultaneously.
In the operation of the device described in the above-named patent, it is sometimes desired to copy a wide sheet of paper, for example, a computer printout sheet, which comes under the influence of both the front aligner rolls and the rear aligner rolls. Since the front aligner rolls attempt to move the side edge of the sheet toward a front reference edge and the rear aligner rolls attempt to move the sheet toward a rear reference edge, the two sets of aligner rolls tend to move the sheet to be copied in opposite directions. In the device described in the above-mentioned patent, it is desired to move the sheet toward the front reference edge for proper positioning, and therefore a slightly higher normal force must be exerted between the aligner rolls at the front than is exerted between the aligner rolls at the rear. By so doing, the large sheet is caused to slip in the nip of the rear aligner rolls and is moved to the front reference edge.
Another problem faced by the SADF of the above-mentioned patent is that the normal force produced between the aligner rolls cannot be too great since the rolls feed the leading edge of the paper against the entry gate. If the force is too high, the paper may buckle or may be damaged. To prevent that result, the normal force between the aligner rolls must be low enough to allow the paper to slip between the rolls when the leading edge reaches the entry gate.
The normal force between aligner rolls cannot be too low, however, since too low a force would cause the paper to slip prior to the time it reaches the entry gate and create a situation in which the paper may not be properly registered at the entry gate.
It has been found that the normal force produced between the aligner rolls must be between 20 and 30 grams in order to drive the paper properly. Within that narrow range of force, the normal force between the front aligner rolls must be greater than the normal force between rear aligner rolls. It is desired, therefore, to produce a front aligner roll nominal force of 271/2 grams with a tolerance of .+-.2 grams and a nominal rear aligner roll force of 221/2 grams with a tolerance of .+-.2 grams. When one considers that the weight of a standard paper clip is about one gram, one can appreciate the sensitive force adjustment needed to hold within the small tolerance levels necessary for the proper operation of the device.
In a roll-mounting device which makes use of a spring to set the required force between rolls, most commercially available springs carry about .+-.10% tolerance in the spring rate. If one desires a .+-.2 gram tolerance in a force of 221/2 grams, it is apparent that the entire tolerance figure for the mounting mechanism is more than consumed by the tolerance in spring rate. Since tolerances must exist in the mechanism itself and in the rolls, the problem is compounded. One should also note that a 10% tolerance in spring rate results in greater than 10% variation in deflection.
In the particular paper moving aligner rolls used in the above-named patent, another problem is the displacement tolerance provided by the nominal positioning of the aligner rolls. Note that one of the aligner rolls is mounted in a movable cover, thus the position of this roll is not determined until the cover is closed. One may appreciate that a significant tolerance must be provided in the positioning of this roll from machine to machine to accommodate the nature of a pivoting cover. In such an environment, it is essential to provide a roll mounting mechanism which is capable of adjusting to a nominal spring rate and capable of providing a relatively constant normal force over a relatively wide deflection range. Additionally, as outlined above, the roll mounting mechanism must achieve these results despite the presence of accumulated tolerances in the roll mounting mechanism itself as well as tolerances in the spring rate.
The object of this invention, therefore, is to provide a mechanism for mounting a backup roll to mate with a drive roll such that a relatively constant normal force is provided between rolls over a wide deflection range.